1. Field of the Invention
This invention relates to a method for detecting a prospective contour point of an irradiation field on a recording medium in order to recognize where an irradiation field lies on a recording medium in the course of reading out a radiation image which has been recorded on the recording medium such as a stimulable phosphor sheet.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as the human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored during exposure to the radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, which is used when the radiation image of the object is reproduced as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
In the aforesaid radiation image recording and reproducing systems, in order to eliminate various problems caused by variations in the image input information and/or to obtain a radiation image having a good image quality which can serve as an effective tool in the efficient and accurate diagnosis of an illness, it is desirable to ascertain the characteristics of the image input information before the radiation image is reproduced as a visible image so that the read-out gain can be adjusted to an appropriate value. The characteristics of the image input information depend on the conditions under which an image is recorded, such as the level of the radiation dose used when the image was recorded, on the image input pattern which is determined by what portion of an object (e.g. the chest or the abdomen of a human body) is recorded and o the image recording method used such as plain image recording or contrasted image recording. Determining the characteristics of the image input information will hereinafter simply be referred to an "ascertaining the image input information". It is also desirable to adjust the scale factor in accordance with the contrast in the image input pattern in order to optimize the resolution of the reproduced image.
The method as disclosed in U.S. Pat. No. 4,527,060 may be used to ascertain the image input information before the visible image is reproduced. In the disclosed method, a preliminary read-out operation (hereinafter referred to as "preliminary read-out") is carried out to ascertain the image input information of a radiation image stored on a stimulable phosphor sheet. During the preliminary read-out stimulating rays are used which have an energy level lower than the energy level of the stimulating rays used in a read-out operation (hereinafter referred to as "final read-out") which obtains a visible image which can be viewed and used, particularly for diagnostic purposes. After the preliminary read-out is completed, the final read-out is carried out. In the final read-out, the read-out gain and/or the scale factor are adjusted to an appropriate value, and/or appropriate signal processing of the image signal is conducted, on the basis of the image input information obtained during the preliminary read-out.
Various methods may be used for approximately ascertaining the image input information of the image stored on the stimulable phosphor sheet from a preliminary read-out image signal obtained during the aforesaid preliminary read-out. One method is to utilize a histogram of the preliminary read-out image signal. Specifically, the image input information can be ascertained from, for example, a characteristic value such as the maximum signal value, the minimum signal value, or a signal value which occurs most frequently, i.e. the signal value corresponding to the maximum value of the histogram. Therefore, a visible radiation image having an improved image quality, which makes it a particularly effective tool in the accurate and efficient diagnosis of illness, can be reproduced by adjusting the read-out conditions such as the read-out gain and the scale factor and/or the image processing conditions on the basis of the histogram.
On the other hand, in the course of radiation image recording, it is often desirable for portions of the object not related to a diagnosis or the like to be prevented from being exposed to radiation. Further, when the object portions not related to a diagnosis or the like are exposed to radiation, the radiation is scattered by such portions to the portion that is related to a diagnosis or the like, and the contrast and resolution are adversely affected by the scattered radiation. Therefore, in many cases, when a radiation image is recorded, the irradiation field is limited to an area smaller than the overall recording region on the stimulable phosphor sheet.
However, in cases where the image input information stored on the stimulable phosphor sheet is ascertained in the manner described above, the problem as described below arises. FIG. 2 is an explanatory view showing the state of a radiation image stored on a stimulable phosphor sheet. With reference to FIG. 2, when an irradiation field B is limited to an area smaller than the image recording region on a stimulable phosphor sheet 103 and a preliminary read-out is carried out over an area markedly larger than the irradiation field B, for example, over the overall region of the stimulable phosphor sheet 103, the image input information actually stored within the irradiation field B is ascertained incorrectly. Specifically, in the aforesaid cases, since parts of the preliminary read-out image signal at regions outside of the irradiation field B are also included in a histogram of the preliminary read-out image signal, the histogram does not accurately represent the actual image input information of the image stored within the irradiation field B.
The applicant has proposed various methods for recognizing an irradiation field as disclosed in, for example, U.S. Patent application Ser. No. 760,862 now U.S. Pat. No. 4,851,678. The proposed methods allow the aforesaid problem to be eliminated by recognizing where the irradiation field lies on the stimulable phosphor sheet, and carrying out the final read-out only on the region thus recognized.
In general, in the disclosed methods for recognizing an irradiation field, several points which are considered to be present on a contour of the irradiation field, i.e. several prospective contour points, are detected. Thereafter, the straight lines or curves connecting the prospective contour points are detected, and the region surrounded by the straight lines or curves is recognized as the irradiation field.
A novel method for detecting a prospective contour point has been proposed in, for example, U.S. Patent application Ser. No. 4,851,678. The proposed method comprises the steps of detecting light emitted by the stimulable phosphor sheet when it is exposed to stimulating rays and thus obtaining an image signal, sampling and digitizing the image signal so that a digital image signal component represents the image information at each position of a predetermined number of positions on the stimulable phosphor sheet, and carrying out differentiation processing of the digital image signal components representing image information stored at positions located along a single line on the stimulable phosphor sheet. Points at which the absolute value of the differentiated values obtained during the differentiation processing exceed a predetermined threshold value are detected as prospective contour points. In cases where several such points are present, the point nearest to an edge of the stimulable phosphor sheet is detected as a prospective contour point. In the proposed method, if digital image signal components carrying information about positions on the stimulable phosphor sheet are obtained, it is necessary to define what area each position on the stimulable phosphor sheet cavers. The areas corresponding to each position may be defined as covering a single picture element. Alternatively, a plurality of picture elements having a predetermined relation to one another, for example, three to five picture elements standing side by side in a predetermined direction, may be defined as the area corresponding to a single position In the former case, the term "digital image signal component at each position" means the image signal components obtained by digitizing the image signal levels at the picture elements which correspond to said positions. In the latter case, the term "digital image signal component at each position" means the image signal components calculated on the basis of the image signal levels at a plurality of the picture elements which are included in the areas corresponding to said position, for example, the digital image signal component obtained by averaging the image signal levels at a plurality of the picture elements which are included in the area corresponding to said position. In the latter case, the value of the digital image signal component at each position is defined by carrying out pre-processing (linear or non-linear filtering) of the image signal levels corresponding to respective picture elements, for example, by carrying out one-dimensional smoothing of the image signal levels corresponding to the respective picture elements located at intervals of three to five lines on the stimulable phosphor sheet.
The type of differentiation processing carried out on the digital image signal components at respective positions may be one-dimensional differentiation of first or higher order, or may be two-dimensional differentiation of first or higher order. If the image has been discretely sampled, differentiation is equivalent to the calculation of the difference between neighboring image signal components.
The values of the aforesaid digital image signal components are proportional to the energy levels of the radiation which impinged upon a stimulable phosphor sheet. Therefore, in general, image signal components outside of an irradiation field have low quantum levels, and image signal components inside of the irradiation field have high quantum levels. Accordingly, in general, the difference between the image signal components corresponding to a portion (an edge portion) of the radiation image containing the contour of the irradiation field has a quantum level higher than the quantum level of the difference between the image signal component corresponding to other portions of the radiation image. A prospective contour point can be detected on the basis of the value of the difference between the image signal components.
The method, wherein a prospective contour point of an irradiation field is detected by carrying out differentiation processing as described above, is applicable to an irradiation field of any shape, and is therefore practically advantageous. However, in cases where a comparatively high level of energy from scattered radiation has been stored in a region outside of an irradiation field on a stimulable phosphor sheet, prospective contour points are often detected incorrectly. Specifically, at the region where energy from scattered radiation has been stored, the differentiated values obtained by the differentiation processing described above increase or decrease sharply at a certain point. Such a point is located nearer to the edge of the stimulable phosphor sheet than the points on the contour of the irradiation field. Therefore, such a point is readily detected incorrectly as a prospective contour point. Such problems arise also when a radiation image is read out from a recording medium other than a stimulable phosphor sheet.